A gun battle left at least one person dead outside the presidential residence in Guinea-Bissau, hours after election results were announced.
President Joao Bernardo Vieira phoned his Senegalese counterpart, Abdoulaye Wade, to say that soldiers were firing on his house.
The African Union warned against any "attempt to seize power by force" in a country with a recent history of coups.
The former ruling party won the parliamentary poll by a large margin.
BBC West Africa correspondent Will Ross says that this appears to have been a failed coup attempt.
The city was reported to be calm again on Sunday morning.
In recent years, the former Portuguese colony became a major transit point for South American cocaine headed for Europe.
The international community has been worried that without effective political leadership it is in danger of becoming a narco-state with drug barons holding more power than the politicians and the rule of law, our correspondent says.
Guinea-Bissau's interior ministry said one of the people who had shot at the presidential residence was killed while several guards loyal to the president had been injured.
Witnesses told Reuters news agency of two deaths during the gun battle in the town's Tchon de Pepel district.
President Wade told French radio that Mr Vieira had contacted him overnight.
"I received a telephone call from President Nino Vieira who told me that there were soldiers at the front of his residence who were firing shots and I asked him to be precise," he said.
"He told me that the soldiers were firing at his home."
Mr Wade added that he had raised the issue with the West African regional body Ecowas and the African Union (AU).
AU spokesman El-Ghassim Wane told French radio that the body was "very concerned about the situation", stressing that it rejected "all unlawful change of government".
According to the National Electoral Commission, the African Party for the Independence of Guinea-Bissau and Cape Verde (PAIGC) won 67 of the 100 National Assembly seats.
The party allied to the president, the Republican Party for Independence and Development (PRID), won just three seats in the ballot, which was held last weekend.
The leader of the party which officially finished second with 28 seats contested the result.
Koumba Yala, leader of the Social Renewal Party (PRS) and himself a former president of Guinea-Bissau, said he would "never accept fabricated results".
His party has strong support in the military, our correspondent notes.